1. Field of the Invention
The present invention relates to an image pick-up apparatus provided with a solid-state image sensing device.
2. Description of the Related Art
Video cameras and electronic cameras are very popular today. Various types of apparatus are now commercially available whereas a great number of new types of apparatus are under development. CCDs are used as an image sensing device in most of these apparatus.
FIGS. 7 to 9 illustrate the structure of a conventional CCD and its reading operation, wherein FIG. 7 illustrates the basic structure of the CCD and FIGS. 8 and 9 illustrate waveforms of timing signals used to drive the CCD.
As shown in FIG. 7, the CCD includes: a photoelectric conversion part 20 for converting an optical image of an object to an electric signal (signal charge); a vertical transfer part 21 for transferring the charge of each pixel in a vertical direction; a horizontal transfer part 22 for transferring each charge obtained via the vertical transfer part 21 in a horizontal direction; and an output amplifier 23 for converting the transferred charge to a signal in the form of voltage and then outputting it. An image signal is read from the CCD in different two modes. In a field reading mode (referred to also as a field mode) signals of all pixels are acquired into the vertical transfer part, signals of two pixels adjacent in the vertical direction are added together, and the resultant signals are transferred in the vertical transfer part. On the other hand, in a frame reading mode (referred to also as a frame mode), signals of pixels on odd numbered lines and those on even numbered lines are transferred separately to the vertical transfer part.
FIG. 7b is a cross-sectional view of a pixel of the image sensing device. FIG. 7c illustrates a potential profile, and FIG. 7d is a top view of electrodes. As shown, a vertical overflow drain is employed as an anti-blooming mechanism. Image sensing devices of this type has a color filter including complementary colors arranged in a checkered pattern. During a vertical blanking period, a signal obtained at each pixel by means of photoelectric conversion is transferred to a transfer stage of a corresponding vertical transfer part. Then, signals of each pair of pixels on a vertical line are added and read out. In general, as shown in FIG. 7, the above reading operation is performed in a quasi interlace fashion in which odd field reading is performed by adding pairs of pixels that are shifted by one line from those in the even field reading.
FIG. 8 illustrates waveforms of four phase vertical driving pulses V1 through V4 used in a field mode operation. A reading pulse is superimposed on the vertical driving pulses V1 and V3 every vertical period during a blanking period so that signal charges of odd numbered lines and those of even numbered lines are transferred at the same time to the vertical transfer part. Then, the vertical driving pulses V1-V4 are supplied at intervals of H (horizontal period) so that signals of the odd numbered lines and signals of the even numbered lines are added together in a predetermined manner and the resultant signals are transferred in the vertical direction. The manner of adding signals of the odd and even numbered lines is determined by the first pulse of the phase of the transfer pulses V1-V4 during a vertical period and thus a shift by amount of one pixel occurs every vertical period thereby providing an interlace effect.
FIG. 9 illustrates waveforms of driving pulse V1-V4 in a frame mode operation. In this mode, a reading pulse is added every vertical period. That is, only signals of odd numbered lines are transferred to the vertical transfer part and read out during a vertical period. Then, signals of even numbered lines are transferred to the vertical transfer part and read out during a subsequent vertical period.
In CCDs of the types used widely today, a VOD structure is employed. In this structure, unnecessary charges are swept away into deeper places of a silicon substrate thereby achieving a high sensitivity and a greater dynamic range.
In an image pick-up apparatus provided with a CCD of the above-described type, the quality of a picked-up image is sensitive to the dynamic range of the CCD. If the image sensing device (CCD) used has a narrow dynamic range, saturation occurs in the image signal when taking a picture of an object having high contrast. This results in a change in color of a portion of the image having high color saturation or results in a reduction in contrast of a portion of the image having high brightness.
In CCDs of the type widely used in video cameras, it is assumed that the reading operation is performed in the field reading mode. Therefore, the dynamic range is optimized for the field mode operation. More specifically, the maximum charge that can be stored in a stage of the vertical transfer part is set to twice the maximum charge that can be stored in a photoelectric conversion part.
If such a CCD or image sensing device is operated in a frame reading mode, one pixel of the photoelectric conversion part corresponds to one stage of the vertical transfer part and thus the dynamic range of the CCD output in the frame mode operation becomes smaller than that obtained in the field mode operation, although the vertical transfer part 21 has a high storage capacity. Since the maximum charge capacity of one pixel of the photoelectric conversion part is smaller than that of one stage of the vertical transfer part, saturation can occur in the photoelectric conversion part in the frame mode operation whereas no saturation occurs yet in the transfer part. This results in a lower saturation level in the frame mode than in the field mode.
In the CCDs of the widely-used type, as described above, since the dynamic range of one stage of the vertical transfer part is equal to the dynamic range of two combined pixels of the photoelectric conversion part, the dynamic range in the frame mode operation is as small as half that in the field mode operation.
In particular, in electronic cameras that operate in a frame mode to achieve high resolution and high picture quality, the above-described reduction in the saturation level causes serous degradation in the picture quality. To solve the above problem, image sensing devices have been developed that allow all pixels to be read at the same time.
In image sensing devices of this type, it is assumed that the reading operation is performed pixel by pixel and thus it is possible to avoid the problem arising from the above-described imbalance between the photoelectric conversion part and the transfer part. However, the image sensing device of this type is too expensive to be used in low-cost types of image pick-up apparatus.
Another known technique to solve the problem described above is to apply a lower voltage to the silicon substrate of an image sensing device during a frame mode operation than during a field mode operation thereby expanding the maximum charge storage capacity of the photoelectric conversion part.
FIG. 7c illustrates the potential distribution from the position just under the photoelectric conversion part of the image sensing device to a position in the silicon substrate. In FIG. 7c, the solid line represents a potential distribution obtained when the substrate potential is fixed to Vsub1. In this case, the maximum storage capacity, and thus the dynamic range, of the photoelectric conversion part is determined by the depth of a potential well defined by the level 1 shown in the figure. The broken line represents a potential distribution obtained when the substrate potential is fixed to Vsub2. In this case, the maximum storage capacity is determined by the depth of a potential well defined by the level 2. As can be-seen from the above discussion, the dynamic range of the image sensing device can be expanded by changing the substrate potential from Vsub1 to Vsub2.
Thus, this property of the CCD is used to expand the dynamic range in the frame mode operation by switching the substrate potential from that in the field mode operation.
As described above, if the electrical potential of the substrate of the CCD is switched in such a manner that Vsub1 is applied to the substrate in the field mode operation and Vsub2 is applied to the substrate in the frame mode operation, then the dynamic range of the photoelectric conversion part in the frame mode operation can be expanded from that in the field mode operation so that good balance in the saturation level between the photoelectric conversion part and the vertical transfer part can be obtained in the frame mode operation thereby improving the overall dynamic range of the CCD output.
However, since the above-described technique relies only on the switching of the silicon substrate potential of the CCD, the expansion of the dynamic range is not enough, and thus blooming can still occur for some objects.
It is a general object of the present invention to solve the above problems. More specifically, it is an object of the present invention to increase the saturation charge capacity of an image sensing device without the limitation by the saturation charge capacity of one charge transfer cell of VCCDs wherein the charge transfer cells also act as reading gates.
It is an another object of the present invention to provide an image sensing device having a high saturation charge capacity determined by the sum of saturation charge capacities of two adjacent charge transfer cells of VCCDs, that is, the maximum charge transfer capacity of the VCCDs and thus having a high saturation output level regardless of whether the operation is performed in the field reading mode or in the full frame reading mode thereby expanding the dynamic range of the image sensing device, and thus providing an image pick-up apparatus capable of taking a high-quality picture with a high signal-to-noise ratio.
It is still another object of the present invention to increase the saturation charge capacity of an interline-type CCD used in various applications when operated in the full frame reading mode, thereby increasing the dynamic range when the CCD is used as an image sensing device.
The above problems are solved by the present invention having various aspects and features described below.
According to a first aspect of the present invention, there is provided an image pick-up apparatus comprising:
an image sensing device for converting an optical image into an electrical signal, the image sensing device including a photoelectric conversion part and a vertical transfer part;
mode switching means for switching an operation mode between a frame mode and field mode; and
control means for controlling the bias level and/or a timing of a vertical transfer pulse depending on the operation mode selected via the mode switching means;
thereby expanding the dynamic range and thus preventing relating problems such as blooming.
In particular, it is possible to expand the dynamic range of a CCD even in the frame mode to a desirable level by varying the pulse width and various setting voltages depending on the operation mode. Therefore, a high-quality general-purpose image sensing device can be achieved without adding any special expensive circuits.
According to another aspect of the present invention, the voltage applied to the substrate is also varied depending on the operation mode thereby achieving further expansion of the dynamic range and thus preventing more effectively relating problems such as blooming.
According to still another aspect of the present invention, all levels of pulses used to drive the vertical transfer part are shifted by substantially the same amount so as to expand the dynamic range without reducing the vertical transfer efficiency, and thus without degradation in picture quality.
According to a further aspect of the present invention, circuits for determining the above parameters of pulses applied to the vertical transfer part have a power source used in common by all these circuits, and furthermore all these circuits have similar temperature characteristics thereby minimizing the influence of temperature on the dynamic range and the transfer efficiency.
Furthermore, the voltage applied to the substrate of the image sensing device is also switched thereby expanding the dynamic range further and preventing problems such as blooming.
According to another aspect of the present invention, there is provided an image pick-up apparatus comprising:
a plurality of photoelectric conversion cells;
charge transfer means including charge transfer cells wherein the number of the charge transfer cells is greater than the number of the photoelectric conversion cells; and
control means for controlling the operation of transferring signal charges from the photoelectric conversion cells to the charge transfer means according to a procedure including the steps of: forming a plurality of potential wells by a plurality of charge transfer cells; and transferring a signal charge from each photoelectric conversion cell to a potential well formed at a position corresponding to each photoelectric conversion cell.
According to still another aspect of the present invention, there is provided an image pick-up apparatus comprising:
a plurality of photoelectric conversion cells;
charge transfer means including charge transfer cells wherein the number of the charge transfer cells is greater than the number of the photoelectric conversion cells; and
control means for controlling the operation of transferring signal charges from the photoelectric conversion cells to the charge transfer means according to a procedure including the steps of: forming a potential well in a charge transfer cell disposed at a position corresponding to each photoelectric conversion cell; transferring a signal charge from each photoelectric conversion cell to the potential well formed at the position corresponding to each photoelectric conversion cell; and then immediately applying a predetermined voltage to a transfer cell adjacent to each potential well thereby increasing the capacity of each potential well.
According to further aspect of the present invention, there is provided a method of driving an image sensing device, the image sensing device including a plurality of photoelectric conversion cells and charge transfer means including a plurality of charge transfer cells wherein the number of the charge transfer cells is greater than the number of the photoelectric conversion cells, the method being characterized in that a signal charge is transferred from each photoelectric conversion cell to the charge transfer means according to a procedure including the steps of:
forming a plurality of potential wells by a plurality of charge transfer cells; and
transferring a signal charge from each photoelectric conversion cell to a potential well formed at a position corresponding to each photoelectric conversion cell.
According to another aspect of the present invention, there is provided a method of driving an image sensing device, the image sensing device including a plurality of photoelectric conversion cells and charge transfer means including a plurality of charge transfer cells wherein the number of the charge transfer cells is greater than the number of the photoelectric conversion cells, the method being characterized in that a signal charge is transferred from each photoelectric conversion cell to the charge transfer means according to a procedure including the steps of:
forming a potential well in a charge transfer cell disposed at a position corresponding to each photoelectric conversion cell;
transferring a signal charge from each photoelectric conversion cell to the potential well formed at the position corresponding to each photoelectric conversion cell; and
immediately after the above step, applying a predetermined voltage to a transfer cell adjacent to each potential well thereby increasing the capacity of each potential well.
According to the present invention having various aspects described above, it is possible to achieve a great increase in the saturation charge capacity and thus the saturation output of the image sensing device in both full frame reading mode and field reading mode. This means that it is possible to achieve a video camera capable of taking a high-quality picture both in the field reading mode and in the full frame reading mode.